The present invention relates to a process and system for recycling and reusing gray water, and more particularly, to a process and system which uses hollow-fiber membrane ultrafiltration in a closed-loop water recycle, such as a clothes washers, also redirectable to a storage tank for other purposes. This invention teaches the immediate use of the permeate generated by gray water membrane separation processes for additional cleaning extractions in clothes washers for equal or better cleaning performance, and also teaches the necessary system components for autonomous, long-life, closed-loop separation with a gray water processing apparatus.
For definitional purposes, "gray water" is defined as lightly contaminated waste water generated by dishwashers, washing machines, sinks, showers, carwashes and bathtubs, such water is relatively free from high-level organic contaminants and bacteria typically found in more contaminated "black water". The origin of gray water also insures that a portion of the constituents are surfactants, contained in soaps and detergents which emulsify and bind with organics in the water.
Membrane filtration is defined as the use of polymeric or ceramic substrates in the form of spiral-wound modules or tubes to separate species in a liquid or gas stream according to molecular size and chemical properties. The spectrum of membrane filtration for liquids varies from reverse osmosis (RO) to microfiltration (MF), between which nanofiltration (NF) and ultrafiltration (UF) fall. The spectrum itself is defined in terms of molecular weight cutoff (MWCO) which is a measure of the molecular sizes (i.e., molecular weight of specie) beyond which the membrane will reject. The assigned values for RO, NF, UF, and MF are nominal, and also depend on the membrane material itself. For the purposes of the following discussion, UF will refer to molecular weight cutoff's in the range of 5,000 to 500,000. For example, species such as endotoxins, pyrogens, virus, bacteria, pigments, dyes, red blood cells, emulsified oils and other organics, asbestos, and pollen can be removed by membranes having MWCO's of nominally 5,000 or less.
The permeate of a membrane filtration system is generally defined as the fluid (gas or liquid) which permeates through the membrane structure, and the retentate is the fluid which does not filter through the membrane structure and is therefore swept across, or out of, the membrane cartridge.
The United States is increasingly burdened with high potable water demands and costly downstream water treatment processes. As a practical matter, the costs for water treatment and supply are ultimately borne by the consumer, so that reductions in these costs, where economically feasible, draw strong political and commercialization support. Military, industrial and residential sources generate voluminous quantities of gray water from dishwashers, washing machines, sinks, showers, and bathtubs. Because thirty-two states have recently adopted codes for reuse of gray water, the engineering impetus for developing robust and economical separation strategies is a reality. The private sector is also being pushed by the Department of Energy (DoE) to develop more efficient, low-water, minimal byproducts washers capable of using cold water.
The treatment and recycling of gray water (e.g., carwash, dishwasher, shower, and laundry) has been explored and put into limited practice during the past twenty years for commercial and military applications. The treatment and recycle scheme depends strongly on the size of the application, chemical and physical properties of the gray water, and logistical requirements of the operation. Currently, most gray water recycle applications have been targeted for carwashes and commercial laundromats, utilizing depth filtration and carbon adsorption as described in U.S. Pat. Nos. 4,104,164 and 5,093,012. As an example of a complicated multi-stepped process, U.S. Pat. No. 4,104,164 describes a waste water treatment process for laundry and carwash water making use of sand filters, centrifugal separators, precipitation of surfactants and other organics, and adsorption media. U.S. Pat. No. 3,772,901 also describes removal of phosphates from clotheswasher water; however, phosphates have generally since been phased out of detergent formulas. The primary concern is now a reduction of gray water generation and, hence, fresh water supply demands placed on municipalities and other primary water suppliers.
Others have demonstrated the physical and chemical separation potential of UF for gray water and other waste streams. The system requirements for an autonomous, closed-loop system have not, however, been recognized, and direct re-use of the permeate stream for subsequent extraction cleaning has not been described. For example, McCray et al. Design Of An Ultrafiltration/Reverse Osmosis Prototype Subsystem For The Treatment Of Spacecraft Wastewaters (1995) describe the use of UF having the sole purpose of a prefilter for a reverse osmosis water treatment station aimed at producing potable water. The direct utility of UF permeate for gray water recycle or clothes washer extraction was not recognized or taught. Adamson et al. Reverse Osmosis Treatment of Selected Shipboard Generated Waste Streams (1974) and Lawrence et al. Development of a Reverse Osmosis Module for Wash Water Recycling in a Space Environment, (1974) also describe systems utilizing RO for wastewater and gray water cleanup, but do not teach the use of UF in closed-loop systems, the required prefilters to increase the life of the system, or the direct re-use of membrane permeate since these studies were aimed at producing potable water. Bhattacharyya et al. Membrane Ultrafiltration to Treat Laundry Wastes and Shower Wastes for Water Reuse, (1974) also reports testing UF technologies for gray water component separation, but this separation technology did not involve closed-loop in situ processing of gray water streams such as clothes washers, and the studies did not consider other system requirements and eventual process sequencing of permeate and retentate stream to arrive at a reduction of gray water generation. Lard et al. Shipboard Laundry Wastewater Treatment Systems, (1976) also reports investigating a UF membrane system for gray water processing. Only the separation capability of these membranes was reaffirmed, and a functional system using the separation media in a closed-loop autonomous system with permeate reuse for additional extraction cleaning was not demonstrated. As a final example, Lent, Renovation of Waste Shower Water by Membrane Filtration, (1976) reports considering bulk treatment of shower water and storage of the permeate using UF. The separation capacity of the UF elements was reinforced; however, application of the technology, along with prescription of the necessary prefilters and sequencing for an autonomous closed-loop system which directly reuses the permeated, was not described.
Gray water can be treated, stored and used for lawn watering or toilet flush water as disclosed in U.S. Pat. Nos. 5,317,766; 5,243,719; 5,106,493; 5,059,330; and 4,162,218. With additional "polishing" steps such as UV irradiation (U.S. Pat. No. 5,288,412), or ozonation (U.S. Pat. No. 5,160,606), gray water can be reused for more bacteriological-sensitive water streams as shower or dishwashing. Immediate reuse of shower water, without any chemical treatment or separation process has also been disclosed in U.S. Pat. No. 5,353,448. Carwashes for water recycle are also known, as seen in U.S. Pat. No. 5,093,012, which utilize carbon adsorption and diatomaceous earth filters. A carwash gray water recycle system has also been discussed in U.S. Pat. No. 5,160,430 using reverse osmosis (RO) in which the retentate or concentrate is the wash water, and the permeate of the RO system subsequently rinses the vehicles. This system is not, however, continuous and is, therefore, not automatic (i.e., operator controlled).
A commercial laundry can also treat the wash water via membrane filtration and additional polishing steps for reuse in laundry operations. Membrane filtration is particularly well-suited for gray water recycle schemes because it requires considerably lower pressures than RO, and the looser structure of the membrane is more resistant, in general, to fouling associated with particulates and larger chemical species. Cross-flow membrane filtration also allows for continuous processing of gray water. The use of ultrafiltration, nanofiltration, and microfiltration has advantages over RO in water treatment with decreased potential for fouling (due to particulates and high oil/organic-content streams), and decreases operating costs because RO requires high operating pressures.
Reverse osmosis will reject species on an ionic scale (molecular weights of 200-500 and lower), nanofiltration will reject species on a molecular scale (molecular weights of 200-10,000), ultrafiltration will reject species in the macro-molecular scale (molecular weights of 1,000-500,000), and microfiltration will reject species from molecular weights of 100,000 to 1,000,0000 (pore size of 0.05-1 .mu.m). All four separation categories will reject bacteria, a benefit in recycle systems where bacteria is present. The pressure (i.e., power) and high-pressure plumbing (i.e., system weight) requirements for membrane separation processes decreases from the 400 psia in RO systems to approximately 10-20 psig for MF systems. Aside from the system requirements, the separation results using lower molecular weight cut-off membranes (e.g., RO) may not always be desirable, and the membranes may be more prone to contamination and plugging problems.
An object of the present invention is to provide a complete process and integrated system for recycle and reuse of gray water generated by sources such as clotheswashers, showers, baths, and carwashes without diminishing the cleaning performance of the overall process.
It is also an object of this invention to provide in situ gray water recycling to generate higher-quality water from the gray water source which can subsequently be used for additional cleaning extractions.
Another object of the invention is to allow detergent and builders from the previous wash cycle to be reused, thereby reducing the overall requirements for clothes washing and the chemical quantities required to be processes by municipal water treatment facilities.
It is yet another objective of the present invention to teach the utility of membrane filtration with molecular weight cutoff's greater than those of reverse osmosis for economical permeate re-use for gray water generator processes such as clothes washers.
A still additional object of this invention is to provide process and system requirements from which bacteria, virus, pigments, dyes, blood cells, endotoxins, pyrogens and large-molecular-scale components can be removed in bulk and further processed to a potable quality suitable for re-use in drinking, showering, or dishwashing.
Another object of this invention is to teach the necessary system components and operating parameter for a closed-loop autonomous gray water recycle system, such as a clothes washer, which directly utilizes waste water in the form of permeate water for additional cleaning extractions.
These objects have been achieved in accordance with the present invention by using membrane filtration, namely, ultrafiltration or other membranes having MWCO's in the range of 5,000 to 500,000, and to recycle gray water so as to reuse the permeate directly as a clean-water extraction agent, or indirectly by storage and eventual reuse in applications which can admit lower-quality, non-potable water such as lawn watering, carwashing, and toilet flush water.
An advantage of the present invention is that it can also be used simultaneously with disinfection processes such as those described by in U.S. patent application Ser. No. 08/417,988, filed Apr. 6, 1995 in the names of Nidal A. Samad and Dwight D. Back and entitled WATER DISINFECTION METHOD USING METAL-LIGAND COMPLEXES, the subject matter of which is incorporated by reference herein, or by ozonation, chlorination, or UV irradiation. Membrane filtration facilitates additional chemically-derived disinfection since no by-products are produced as a result of interaction between the gray water separation chemicals and those used for disinfection such as ozone and chlorine.